The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al2O3 composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al2O3. The interconnected Al2O3 in the composite material is separated by Sigma 3 boundaries (twins) with a 60 degrees rotation around the [0001] direction. A high frequency (similar to 100%) of Sigma 3 coincidence boundaries in composite alumina is remarkable since only similar to 12% of boundaries in a sintered polycrystalline Al2O3 are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.